A four-wheel drive transmission of the type set forth above which is considered to be of the simplest structure in that, as disclosed in JP, U No. 3-73751, there is provided at a front side of the hydrostatic transmission a front-wheel drive assembly which comprises a front-wheel drive shaft for taking-off front-wheel driving power and a front-wheel drive clutch for selectively connecting the drive shaft to the front end of an output shaft of the hydrostatic transmission. However, this structure cannot be adopted when the rear-wheel drive assembly includes a speed change mechanism. This is because, while the peripheral speed of rear wheels is changed in response to a shifting operation of the speed change mechanism, the peripheral speed of front wheels driven to rotate through the front-wheel drive shaft remains unchanged, so that a condition may be caused where the peripheral speed between front wheels and rear wheels differs greatly.
Accordingly, in a transmission in which the rear-wheel drive assembly includes a speed change mechanism, a front-wheel drive shaft is provided which is connected to a transmission shaft of the driven side of the change mechanism through a clutch. Four-wheel drive transmissions each comprising such a front-wheel drive shaft are disclosed in U.S. Pat. No. 4,579,183; U.S. Pat. No. 4,373,597; and JP, U No. 3-51260.
Of these transmissions according to the prior art, the transmission disclosed in U.S. Pat. No. 4,579,183 comprises a hydrostatic transmission mounted on a front surface of a transmission casing, in which a rear-wheel drive mechanism is housed, and a front-wheel drive shaft which extends forwardly from the transmission casing at a level below that of the hydrostatic transmission. Although this structure has an advantage in that the front-wheel drive shaft can be provided without making any change or modification in a standard structure of the hydrostatic transmission, such front-wheel drive shaft is located at a low level so that it provides a restriction to the inclusion of a mid-mount auxiliary implement, such as a mid-mount mower, which is disposed at an underside of the vehicle.
In the transmission disclosed in U.S. Pat. No. 4,373,597, a front-wheel drive shaft extends forwardly from a transmission casing and through the housing of a hydrostatic transmission which is mounted on the front of the transmission casing. Accordingly, the front-wheel drive shaft is located at a high level. However, because this drive shaft must be arranged such that it does not interfere with the hydraulic pump and motor of the hydrostatic transmission, location of the drive shaft is deviated in the lateral direction of the vehicle to the left or right side so that arrangement of even a front-axle casing into which power is transmitted from the drive shaft through a transmission shaft must be changed. Further, the additional of structure to the housing of a hydrostatic transmission for passing the drive shaft therethrough is not preferred because such structure will make the hydrostatic transmission a design that is not a standard design and, therefore, will increase cost.
In the transmission disclosed in JP, U No. 3-51260, the output shaft or motor shaft of a hydrostatic transmission which is mounted on the front of a transmission casing is made hollow, and a front-wheel drive shaft is provided such that it extends forwardly from the transmission casing and through the hollow motor shaft. Although this structure also makes the level of the front-wheel drive shaft appropriate, such structure will make the hydrostatic transmission a design that is not a standard design and, therefore, will increase cost considerably. This is because the hollow motor shaft requires an enlargement of the outer diameter of this shaft so that the structure of hydraulic motor itself must be changed substantially.
Accordingly, a primary object of the present invention is to provide a four-wheel drive structure in a transmission comprising a rear-wheel drive assembly, which includes a two-stage speed change mechanism having high-speed and low-speed positions, disposed at a rear side of the output shaft of a hydrostatic transmission, by incorporating a front-wheel drive assembly at a front side of the output shaft of the hydrostatic transmission so as not only to make the level of a front-wheel drive shaft appropriate but to reduce cost.
In view of the fact that a working vehicle such as a tractor is used for various working operations and the fact that such vehicle is generally operated at a low speed when it is used for a working purpose and at a high speed when a particular tractive force is not required, e.g., when it is travelled on a road, another primary object of the invention is to provide a four-wheel drive structure, in a transmission including a two-stage speed change mechanism for rear wheels, which can provide plural drive modes for front wheels.